Polymer blends comprising a second polymer dispersed in a matrix of a first polymer are very useful and, depending on the properties and the relative amounts of the first and second polymers, a wide variety of such polymer blends can be produced. Of particular interest are polymer blends, also referred to as thermoplastic elastomers, in which the first polymer is a thermoplastic material, such as polypropylene, and the second polymer is an elastomeric material, such as an ethylene-propylene elastomer or an ethylene-propylene-diene monomer (EPDM) rubber. Examples of such thermoplastic elastomers include polypropylene impact copolymers, thermoplastic olefins and thermoplastic vulcanizates.
Unlike conventional vulcanized rubbers, thermoplastic elastomers can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures. For this reason, thermoplastic elastomers are useful for making a variety of articles such as weather seals, hoses, belts, gaskets, moldings, boots, elastic fibers and like articles. They are also particularly useful for making articles by blow molding, extrusion, injection molding, thermo-forming, elasto-welding and compression molding techniques. In addition, thermoplastic elastomers are often used for making vehicle parts, such as but not limited to, weather seals, brake parts including, but not limited to cups, coupling disks, diaphragm cups, boots such as constant velocity joints and rack and pinion joints, tubing, sealing gaskets, parts of hydraulically or pneumatically operated apparatus, o-rings, pistons, valves, valve seats, and valve guides.
One particularly useful form of thermoplastic elastomer is a thermoplastic vulcanizate (“TPV”), which comprises a thermoplastic resin matrix, such as polypropylene, within which are dispersed particles of a cured elastomeric material, such as an EPDM rubber. TPVs are normally produced by a process of “dynamic vulcanization”, which is a process of vulcanizing or cross-linking the elastomeric component during intimate melt mixing with the thermoplastic resin, together with plasticizers (e.g. process oils), fillers, stabilizers, a cross-linking system and other additives, under high shear and above the melting point of the thermoplastic polymer. The mixing is typically done in a twin-screw extruder, to create a fine dispersion of the elastomeric material within the thermoplastic resin while the elastomeric material is cured. The levels of thermoplastic resin and plasticizer (oil) can be adjusted to produce grades having different profiles of hardness, rheology and engineering properties, although in general it is difficult to produce TPVs by dynamic vulcanization in which the content of the elastomeric phase is greater than 50 wt % of the overall polymer blend.
However, while dynamic vulcanization is effective in producing TPVs with a unique profile of properties, it is expensive and suffers from a number of disadvantages. Not only is the reactive extrusion expensive; finishing, packing, transportation, and handling of both thermoplastic resin and elastomers also add to production cost, which limits the development of value-added products. Thus the production of quality product is technically challenging and specialized equipment is needed. Moreover, the process involves many steps, each one contributing to the eventual quality of the final product. Forming the polymer blend normally involves separately comminuting bales of the elastomeric polymer (which is typically how EPDM rubber is commercially distributed), mechanically mixing it with the thermoplastic resin along with the processing oils, curatives, and other ingredients in a suitable high shear mixing device to comminute the rubber particles and cure them to generate cured rubber particles embedded in a continuous thermoplastic resin matrix. The cured rubber particles in the finished products typically have an averaged particle size of 1 to 10 microns. Careful injection of processing oil helps manage the rheological characteristics of the fluid in the reactive extruder (to minimize pressure buildup) as well as product properties such as hardness. Precise control over the size and distribution of the cross-linked elastomer particles is sought, as it affects properties such as elastic recovery (as measured through compression set). While the products produced with existing technology have many desirable properties, there are gaps in the overall properties profile. Some of these are the need for smaller particle sizes of dispersed rubber within the plastic matrix, higher service temperatures, improved elastic recovery, softer products, higher tensile strength, easier processability, and/or oil-free compositions.
Thus, it would be beneficial to the TPV industry to have a TPV with ultra-fine morphology while still maintaining or preferably exceeding the desirable properties of the dynamic vulcanization in an extruder as mentioned above. It would also be advantageous for the TPV production to be performed in conjunction with the elastomer production without the need to finish and re-dissolve the elastomers. This invention addresses these needs.
GB Patent 1,599,164 discusses the blending of polypropylene (PP) and EPDM in solution. There is no mention of the addition of curatives to enhance physical properties.
A process for producing TPVs is disclosed in U.S. Pat. No. 6,388,016, in which a polymer blend is produced by solution polymerization in series reactors employing metallocene catalysts and the resultant blend is subjected to dynamic vulcanization. In particular, the process involves feeding a first set of monomers selected from ethylene and higher alpha-olefins, and a solvent, to a first continuous flow stirred tank reactor, adding a metallocene catalyst to the first reactor in an amount of 50 to 100 weight % of the total amount of catalyst added to all reactors, operating the first reactor to polymerize the monomers to produce an effluent containing a first polymer, feeding the effluent from the first reactor to a second continuous flow stirred tank reactor, feeding a second set of monomers selected from ethylene, higher alpha-olefins and non-conjugated dienes, and optionally additional solvent, to the second reactor, operating the second reactor to polymerize the second monomers to produce a second polymer containing diene, recovering the resulting first and second polymers and blending them with a curing agent under conditions of heat and shear sufficient to cause the blend to flow and to at least partially cross-link the diene-containing polymer and form a dispersion of cured diene-containing particles in a matrix of the first polymer. It will, however, be seen that this improved process still relies on dynamic vulcanization to cure the elastomeric component. As a result the cured diene-containing particles have an average particle size in the range of 1 to 10 microns.
U.S. Pat. No. 6,245,856 describes the use of a compatibilizer in a PP/EPDM thermo plastic olefin (TPO). In the absence of a compatibilizer, the elastomer dispersion is uneven, with some particles greater than 5 micrometers in size. In the presence of the compatibilizer, the dispersion displayed a particle size of about 1 micrometer.
U.S. Pat. No. 6,579,944 discloses a PP/EPDM based TPV composition with a majority of the particles less than 5 micrometers in size, some as large as 10 micrometers, others less than 0.1 micrometers.
U.S. Pat. No. 4,311,628 provides an example of a PP/EPDM based TPV having an EPDM content of 85% by weight. The disclosure states that the TPV is processable as a thermoplastic polymer.
U.S. Pat. No. 6,498,214 discloses thermoplastic polyolefin blend compositions useful for soft touch TPO applications which comprise, in weight percent based upon the weight of components A and B. A is about 99 to about 1 percent of at least one polypropylene homopolymer or copolymer; and B is about 99 to about 1 percent of at least one homogeneously branched ethylene/alpha-olefin copolymer in which the alpha-olefin contains at least 4 carbon atoms and the homogeneously branched copolymer has a crystallinity of less than about 3 percent.
U.S. Ser. No. 60/693,030 filed Jun. 22, 2005, discloses a heterogeneous polymer blend comprising: (a) a continuous phase comprising a thermoplastic first polymer having a crystallinity of at least 30%; and (b) a dispersed phase comprising particles of a second polymer different from the first polymer dispersed in said continuous phase, the second polymer having a crystallinity of less than 20% and being at least partially cross-linked, and the average particle size of the particles of the second polymer being less than 1 micron. U.S. Ser. No. 60/693,030 also discloses a process for producing a heterogeneous polymer blend comprising (a) a continuous phase comprising a thermoplastic first polymer that is at least partially crystalline; and (b) a dispersed phase comprising particles of a second polymer different from the first polymer dispersed in said continuous phase, the second polymer having a crystallinity less than that of the first polymer and being at least partially cross-linked, the process comprising: (i) polymerizing at least one first monomer to produce a thermoplastic first polymer that is at least partially crystalline; and (ii) contacting at least part of said first polymer with at least one second monomer and at least one polyene under conditions sufficient to polymerize said second monomer to produce said second polymer and simultaneously cross-link said second polymer such that the dispersed phase produced by said contacting (ii) comprises at least a fraction which is insoluble in xylene.
Other references of interest include: U.S. Pat. Nos. 3,629,212, 4,016,342, 4,306,041, 6,245,856, 6,207,756, 6,319,998, 6,770,714, 4,130,535, 4,311,628, 6,388,016, and Wu, Souheng, Phase Structure in Polymer Blends: a Criterion for Rubber Toughening, POLYMER, (1985) 26 (12), 1855-63.
In addition, this invention is related to U.S. Ser. No. 60/693,030, filed Jun. 22, 2005, U.S. Ser. No. 11/296,842, filed Dec. 7, 2005, U.S. Ser. No. 11/296,830, filed Dec. 7, 2005, U.S. Ser. No. 11/295,927, filed Dec. 7, 2005, U.S. Ser. No. 11/295,930, filed Dec. 7, 2005, U.S. Ser. No. 60/699,663, filed Jul. 15, 2005.